This invention relates to an optical apparatus, more particularly to a multi-beam optical scanning apparatus for performing scanning using plural beams, which is applicable to an image forming apparatus such as a color printer of a tandem type, a color copy machine of a tandem type, a high-speed laser printer and a digital copy machine, which have plural photosensitive drums.
Image forming apparatuses, such as color printers of a tandem type or color copy machines of a tandem type, which have plural photosensitive drums, each incorporate image forming sections corresponding to color components resulting from color separation, and an optical scanning apparatus, i.e. a laser exposure apparatus, for emitting plural laser beams associated with image data items for corresponding color components.
It is known that those image forming apparatuses include ones which have plural optical scanning devices associated with corresponding image forming sections, and ones which have a single optical scanning device adapted to emit plural laser beams to corresponding image forming sections.
In general, the optical scanning apparatus comprises plural semiconductor laser elements as a light source; a first lens group, i.e. a pre-deflection optical system, for converging beams emitted from the laser elements, to a predetermined diameter; a light-deflecting unit for continuously reflecting the beams converged by the first lens group, to a predetermined portion (i.e. an image surface) of a recording medium which is situated in a main scanning direction perpendicular to a sub-scanning direction in which a recording medium is transferred; and a second lens group, i.e. a post-deflection optical system, for converging the laser beams deflected by the light-deflecting unit so that the image surface of the recording medium will coincide with a line on which the laser beams are actually converged.
The pre-deflection optical system of the optical scanning apparatus includes a semiconductor laser, and a finite focus lens for converging a divergent beam emitted from the semiconductor laser, to a predetermined diameter, or a collimator lens for converting the laser beam to a parallel beam.
When the image forming apparatus with the optical scanning apparatus is used at a high temperature or a low temperature, or when the temperature of the image forming apparatus rises due to the heat generated therein, each lens in it may well be influenced by the heat. In this case, it is possible that the refraction index of the lens will vary to thereby vary its focal distance. A change in focal distance, due to a change in the refraction index of the finite focus lens or collimator lens for converging the divergent beam emitted from the semiconductor laser or converting it to a parallel beam, will increase the beam-spot diameter on a photosensitive drum incorporated in the image forming apparatus, thereby degrading the quality of an image formed on the drum.
In other words, the laser beam emitted from each semiconductor laser is usually converged by a group of lenses to a minimum diameter in an image formation position, i.e. on the photosensitive drum. However, if the focal distance of each lens increases or decreases to shift the focus from the image formation position, the beam-spot diameter on the photosensitive drum will increase, with the result that an image of a high resolution and quality cannot be obtained.
Moreover, in a multi-beam optical system for simultaneously deflecting plural laser beams, the corresponding light sources, finite focus lenses or collimator lenses are located in respective positions of different temperatures. Accordingly, the focal distance of each lens varies due to a change in ambient conditions, in particular, temperature, and hence the beam-spot diameters on the photosensitive drum will vary. As a result, the properties of the optical system will degrade, thereby degrading the quality of an image formed.